We study the molecular and cellular mechanisms that govern the behavior of mammalian neural stem (progenitor) cells, in particular how they balance the competing needs of self-renewal and differentiation, by using the embryonic central nervous system in mice as a model system. We are particularly interested in asymmetric cell division, a process by which a cell divides to produce two different daughter cells. Such divisions play a critical role in generating cellular diversity in invertebrates. We have postulated that mammalian neural progenitor cells balance self-renewal and differentiation by dividing asymmetrically to produce a daughter progenitor cell and a neuron and that in such divisions, mammalian Numb proteins, m- Numb and Numblike (Numbl), asymmetrically segregate to the progenitor daughter to promote its fate. Through research funded by the National Institutes of Health in the last four years, we demonstrated that m-Numb and Numbl are redundant but essential for maintaining neural progenitor cells and that asymmetric Numb segregation and, therefore, asymmetric cell division are indeed critical for neurogenesis to proceed normally during mouse embryogenesis. As a natural extension of these findings, this renewal application seeks to elucidate the molecular pathway that enables Numb proteins to specify cell fates during mammalian neurogenesis. Specifically, we will perform a series of gain- and loss-of-function studies using mice and Drosophila to examine a novel mechanism by which the Golgi apparatus coordinates the timing of cell-fate determination during the progenitor cell cycle by releasing a Numb-binding protein, MERRY MAGPIE (MRGI), to activate Numb signaling only during mitosis to distinguish the two daughter cells. The aims of this application are (1) to test the hypothesis that MRGI and Numb proteins are essential partners in cell-fate specification, (2) to examine the importance of MRGI release from the Golgi in Numb signaling, (3) to determine the precise roles that MRGI plays during mouse neurogenesis, and (4) to assign function to different MRGI domains. If successful, the proposed research may reveal some of the most fundamental mechanisms governing developmental neurobiology and the biology of neural stem cells. Such knowledge may point to mechanisms that cause brain cells to revert to less differentiated states in brain tumors or degenerate in neurological disorders.